CIE AS Biology 4.3: Cell Transport Methods

Diffusion: the net movement of molecules from an area of high concentration to an area of low concentration down a concentration gradient

Facilitated diffusion: diffusion of polar molecules and ions using proteins

Osmosis: the net movement of water molecules from an area of high water potential to an area of low water potential down a water potential gradient

Active transport: the net movement of molecules and ions through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration (ATP) against a concentration gradient

Diffusion

Molecules that can move across the phospholipid bilayer by diffusion are:

• small, uncharged, non-polar molecules
• water
• hydrophobic molecules

In the case of non-polar and hydrophobic molecules, due to their nature, they are able to pass through the hydrophobic core of the phospholipid bilayer. In the case of water and small molecules, the partial permeability of the cell membrane allows them to pass due to their size.

Diffusion occurs to form an equilibrium where the substance concentration inside the cell is equal to the substance concentration outside the cell.

Facilitated Diffusion

Molecules that move across the phospholipid bilayer by facilitated diffusion are:

• large, polar molecules
• ions

Due to their hydrophilic nature, they cannot pass through the hydrophobic core of the phospholipid bilayer. Instead, they need to use proteins (channel and carrier proteins) to enter a cell.

• Channel proteins - water-filled pores, fixed shape; passive transport
  • ‘gated‘ - remains closed on both ends
  • molecules pass freely when open
• Carrier proteins - specific to one type of molecule or ion, can change shape; active and passive transport
  • open on one side, closed on the other side
  • the molecule binds specifically to the protein

Osmosis

Only water moves by osmosis.

Water potential is the term to describe the tendency of water to move in and out of a solution.

Εquation for water potential

ψ = ψs + ψp

water potential = solute potential + pressure potential

where ψs (solute potential) is the reduction in water due to the presence of solutes and ψp (pressure potential) is the hydrostatic pressure to which water is subjected. ψs is always negative and ψp is always positive.

A solution with solutes will always have a negative water potential as pure water has a water potential of 0 kPa at atmospheric pressure.

The closer the water potential value is to 0, the higher the water potential - the more dilute the solution is.

In plant cells, the process of water moving into a cell by osmosis makes the cell turgid. The process of moving water out of a cell by osmosis plasmolyses the cell.

Active Transport

Molecules that move across the phospholipid bilayer by active transport are

• large molecules
• ions
• sugar
• inorganic ions

In the case of active transport, these molecules are important for the reabsorption of useful molecules needed for cell function. They are transported by carrier proteins and use ATP to move against the concentration gradient.

Bulk Transport

Larger molecules may need to be transported in a different way from active transport. This is known as cytosis. Molecules that move across the phospholipid bilayer by cytosis are:

• proteins
• polysaccharides
• parts of cells
• whole cells e.g bacteria

There are two types of cytosis: endocytosis and exocytosis

• Endocytosis - engulfing of materials into a cell using an endocytotic vacuole
  • pinocytosis - uptake of fluids
  • phagocytosis - uptake of solids; phagocytes are specialised cells; form phagocytotic vacuoles
• Exocytosis - secretion of materials out of a cell using a secretory vesicle
  • substances such as enzymes, hormones, or cell wall building materials are secreted
  • secretory vesicles are made from the Golgi body
  • secretory vesicles fuse with the cell membrane and release their contents

Factors that Affect the Rate of Diffusion

• ‘steepness‘ of the concentration gradient
  • ‘steepness‘ refers to the difference in concentration of a substance on two sides of the membrane
  • the greater the difference in concentration, the more molecules will randomly move across the membrane
• temperature
  • higher temperatures give molecules and ions more kinetic energy → move faster
• surface area
  • when there is a greater surface area, diffusion occurs at a higher rate
  • this can be increased by folding e.g microvilli, cristae in mitochondria
  • a high surface area to volume ratio means substances can diffuse more easily than a cell with a low surface area to volume ratio
• properties of ions and molecules
  • smaller molecules will diffuse more quickly than a larger molecule
  • non-polar molecules can diffuse directly across the hydrophobic core of the phospholipid bilayer

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